Generally, installations for producing relatively smaller volumes of separated air components, namely units processing less than 100 tons of product per day, are not cost effective when designed with the two sets of tandem compressor and expander used in large volume installations, namely above 100 tons per day and up to 1,000 tons per day.
In U.S. Pat. No. 4,152,130, an installation is disclosed which utilizes two sets of tandem compressors and expanders to supply refrigeration for the separation of air into its major components, nitrogen and oxygen. This installation operates in the over 100 ton per day category.
U.S. Pat. No. 3,492,828 discloses an installation for the separation of gas mixtures wherein a single tandem compressor and expander is utilized to cool a feed gas stream by indirect heat exchange rather than by direct expansion of the gas feed stream. Additional expansion valves and heat exchangers are utilized for supplemental refrigeration.
U.S. Pat. No. 3,091,094 teaches the utilization of a split-out stream from a heat exchange unit in an air separation installation. The split-out stream is not utilized to further refrigerate the feed air stream of the installation.
U.S. Pat. No. 3,079,759 discloses an air separation unit wherein a portion of the feed air stream is split out from the main heat exchanger and refrigerated by expansion through an expander prior to introduction into a distillation column. Auxiliary freon refrigeration is not set forth.
In an article authored by R. E. Lattimer entitled "Distillation of Air" appearing in Chemical Engineering Progress, Volume 63, No. 2, pages 35-59, February, 1967, various air separation units are disclosed which utilize main-line freon refrigeration units. The freon refrigeration units of this disclosure operate directly to cool the entire main feed air stream and do not operate on a split out stream or in a recycle heat exchange relationship.
Therefore, it is an object of the present invention to provide the necessary refrigeration of the feed air stream to an air separation unit of relatively smaller capacity, wherein the refrigeration is derived from air stream expansion means as well as direct in-line freon refrigeration means on a split-out stream of the feed air stream; wherein refrigeration is performed on at least a portion of an air stream without indirect heat exchange or the use of secondary heat exchange fluids. This invention is directed to air separation in the range of 20 to 100 tons per day (T/D) of liquid product and preferably 30 to 60 T/D.